A gun, like an automobile engine or gas turbine, is a heat engine. The function of a gun as a heat engine is to convert the chemical energy of the propellant into kinetic energy in the projectile. As with any other heat engine, the efficiency of the thermodynamic process in a gun determines how much propellant is required to deliver the required kinetic energy to the projectile. Since the efficiency of a thermodynamic process is measured by the temperature drop across the process (exclusive of losses), then the greater the temperature drop, the greater the efficiency and the smaller the amount of propellant required to launch a projectile at a given muzzle velocity. Temperature drop across the process corresponds directly to pressure drop. Therefore, the greater the pressure drop, the greater the thermodynamic efficiency, resulting in less propellant being required to perform a given amount of work. Small arms firearms have been designed to operate at higher and higher pressures as discoveries and inventions permit to achieve greater efficiencies.
Major enhancements in the performance of small arms internal ballistics have been stalled since the end of the nineteenth century indirectly due to the persistent use of conventional Boxer and Berdan primers. Conventional primers, which are cheap, small, reliable and effective, are well matched for use in the conventional pressure (60,000 psi) cartridges and firearms for which they were designed. However, the design of conventional cartridges has prevented the harnessing of a large percentage of the potential energy contained in propellants. The placement of the primer in the base of conventional cartridge cases locates the primer behind the chamber of the barrel during firing. The cartridge case itself must therefore provide its own radial support in containment of the firing pressure inside the primer pocket.
The use of conventional primers located in the bases of conventional cartridges results in firearms operating at relatively low pressures as compared to the high pressure (230,000 psi) and high efficiency which conventional propellants are capable of delivering. Accordingly, the situation has developed, and has been taken for granted, that large quantities of propellant contained in large “bottle necked” cartridge cases are required to provide currently accepted external ballistics. These large bottle necked cartridge cases have dictated the limits on the kinds and sizes of mechanisms which can be employed in self powered firearms. For example, military bottle necked cartridges with their large diameter bases place very high loads on locking mechanisms because of the large pressure area of the head of the cartridge case. Conventional firearm locking mechanisms must be designed to be much more robust than if their cartridges could be designed with small head diameters.
In self-powered firearms, some of the energy generated by firing is stored in the operating mechanism in the form of kinetic energy, which is subsequently used to power the firearm cycle of functioning. The pressurized gas generated in firing is an excellent power source, but the energy release occurs in a few milliseconds, and then subsides before the energy is needed to perform the work of cycling the firearm.
Several basic methods have been employed for storing functioning energy in conventional firearm operating systems. The most widely employed operating system type used in high powered, light-weight military small arms is gas operation. In typical gas operating systems, a small quantity of propellant gas is directed from the barrel bore into a gas cylinder through a gas port connecting the barrel bore with the gas cylinder. The pressurized gas can be trapped in a variety of piston and cylinder arrangements, and the energy of the trapped gas is then used to accelerate (impart kinetic energy to) the firearm operating mechanism parts. The breech of a gas operated system remains locked and sealed during, and for a short time after, firing. The potential energy (in the pressurized gas) that has been transferred to the gas system is converted into kinetic energy in the operating system primary mass. The primary mass is usually called the operating rod or bolt carrier.
The secondary mass (the bolt) remains locked and stationary while the barrel and cartridge case are pressurized during the time the projectile remains in the bore. After the projectile exits the muzzle and the pressure in the barrel substantially subsides, and after the primary mass moves a short distance (referred to as “dwell”), then the bolt is unlocked through interaction of the bolt carrier (primary mass) with the bolt. After dwell some energy is expended in unlocking, and considerable energy is expended in momentum transfer in picking up the bolt and causing the bolt to move rearward with the primary mass. If the primary/secondary mass ratio is 5/1 the energy loss is 16.8%. If the primary/secondary mass ratio is 4/1, the energy loss is 20%.
Gun designers exercise care in establishing the ratio between the primary and secondary masses. On the one hand, a high primary/secondary mass ratio is desirable in order to reduce the velocity of the bolt carrier impacting and picking up the bolt because the direct impact of highly loaded parts tends to damage parts. On the other hand, a high primary/secondary mass ratio is undesirable because it increases firearm size and weight. Usually the bolt (secondary mass) is designed to be as light as possible while still being able to reliably perform its work. After determining the required bolt weight, the primary mass parts are ordinarily designed with enough mass to provide the minimum acceptable primary/secondary mass ratio while considering the required cyclic rate, and acceptable recoiling mass velocities.
Operating systems which employ a primary/secondary mass are typically costly to manufacture depending upon the number, complexity, fit and material of the parts employed. Moreover, a typical gas operating system requires expensive precision fits and alignment between the gas piston and gas cylinder. A further costly aspect in the production of gas operated systems concerns headspace. Practically speaking headspace is the distance from the face of the fully locked bolt to the rear of a fully seated cartridge. Headspace must be limited to a few thousandths of an inch for a conventional firearm to function reliably and safely. The locking lugs of the bolt, along with their supporting recesses in the receiver, and the chamber, must all precisely fit with each other; i.e. provide proper headspace, to insure the conventional cartridge will be properly positioned and supported during firing. Conventional cartridges used with locked systems must also be precisely manufactured to fit the headspace length of the firearm in order to prevent chambering stoppages if the cartridge is too long; or to prevent case head separations during firing if the cartridge is too short.
A typical locked breech gas operated powering system includes multiple parts and assemblies. Most gas operating system parts, such as gas cylinders, gas pistons, bolt carriers, bolt cam pins, bolts, barrel extensions and receivers must be fabricated to close tolerances. Some parts, such as cams, require complex and expensive machining. Certain features of these parts also require high finishes and close fits with tight tolerances, and must maintain dimensional stability through the heat treatment process. Parts warpage in heat treatment causes many quality assurance problems.
Recoil operation is another type of locked firearm operating system widely used in small arms. Recoil operated systems, like gas operated systems, employ primary/secondary masses with many of the same design considerations as gas operated systems. Recoil operated systems are inherently the least ballistically accurate of the operating systems because the barrel recoils within the receiver, and all the firing parts move relative to the sights.
Retarded blowback operating systems are not locked, but employ primary/secondary masses or toggle arrangements with design considerations similar to those of gas and recoil operating systems. Retarded blowback operating systems are sensitive to mounting conditions and to ammunition variations.
Delayed blowback operating systems remain locked until chamber pressure drops somewhat before the bolt is unlocked and blown back by residual chamber pressure. Delayed blowback operating systems are difficult to design because of the very close timing requirements for unlocking, and their extreme sensitivity to ammunition variations.
Piston primer operation is another type of operating system (see U.S. Pat. No. 3,855,900 to Barr et al.) in which a special piston primer is used. The piston primer functions as the primer, as part of the operating system, and as a sliding seal with the rear of the cartridge to prevent leakage of pressurized propellant gas. The piston primer is driven rearwardly (while maintaining the seal) by the pressurized gas created upon firing. The rear of the piston primer drives rearward the firing pin, which is a part of the primary mass. Piston primer operation, though it eliminates a gas system in the firearm, still requires the same basic primary/secondary mass relationship as required with gas, recoil and retarded blowback operation. All the functions of locking, firing, unlocking, extraction, ejection, and powering are concentrated in and competing to occupy a very small space at the front of the bolt.
Blowback (straight blowback) operation is the simplest of the self-operating firearm systems. Blowback operation is very successfully employed with many low pressure cartridges, especially .22 caliber rimfire cartridges and virtually all sub-machineguns employing pistol cartridges. In blowback operation, there is only a primary mass, the bolt. The bolt does not lock the cartridge into the chamber for firing. Rather, the projectile is accelerated through the barrel by the full force of the propellant gas pressure at the same time the bolt is accelerated rearwardly by the full force of the propellant gas pressure. Only the inertia of the mass of the bolt is required to prevent the bolt from opening too quickly. The restraining effect on the bolt by the operating spring is negligible. Conventional blowback operation is highly desirable for its simplicity and low cost of manufacture. However, blowback operation has been heretofore limited to use with low pressure cartridges in which the entire cartridge case can slip rearward relative to the chamber while the pressure is still being applied to accelerate the projectile through the bore.
The head of a conventional cartridge case, regardless of the firearm operating system employed, acts as the plug for the chamber of the barrel. The cartridge case wall adjoining the cartridge case head seals this plug through expansion of the cartridge case wall against the chamber of the barrel. Since the primer of conventional cartridges is located outside the rear of the barrel breech, firearm operating pressures have been limited by the strength of the case head material surrounding the primer pocket, regardless of the operating system employed and robustness of the firearm.
One problem in employing simple blowback operation in a firearm firing conventional high pressure bottle-neck cartridges is that the pressure in the cartridge case drives the head of the cartridge case and bolt much farther than the cartridge case can stretch while the cartridge case wall is seized in the chamber. In this situation, the cartridge case head will be ripped from the cartridge case body when the firearm is fired, causing the cartridge to rupture.